Astron. Astrophys. 334, 558-570 (1998)

A. Appendix: details on photometric data sets used

A journal of photometric observations is presented in Table A1. Table A2 contains the values of the standard magnitudes of the comparison and check stars used by various observers which I adopted to bring the data onto a comparable scale. To transform Cousins & Warren's (1963) magnitudes into Johnson B magnitudes I adopted the correction of 0 21, recommended by the authors. Moreover - for the purpose of the study of long-term changes only - I also created approximate V magnitudes from these observations, using V = B + 0 18, following again a recommendation of the authors.

Table A1. Journal of photometric observations of  CMa

Table A2. Comparison and check stars and their standard magnitudes adopted here

The uvby data obtained at ESO come from two different sources: Baade's (1982b) observations, which were kindly put at my disposal by the author, and the measurements secured in the framework of the "Long-term Photometry of Variables" (LTPV) project which was initiated more than a decade ago (Sterken 1983, 1994). This latter set consists of 106 datapoints (nightly averages of 1-3 measurements) and its latest edition was kindly put at my disposal by Dr. C. Sterken. The details on the data reduction and transformation to the standard system can be found in Manfroid et al. (1991, 1994) and Sterken et al. (1993, 1995). I used the magnitude differences  CMa - HD 56876 to which I added the magnitudes of HD 56876 listed in Table A2. The calibrated Strömgren indices of HD 56876 are based on data from "System 7" (Sterken 1993) only.

The observations of  CMa from the ESO long-term program have been analyzed by Mennickent, Vogt & Sterken (1994) who reported a very unusual light decrease, especially pronounced in b and v, between JDs 2446479 and ...499. Thanks to the fact that Dr. Sterken provided me with a complete documentation of their observations, I could recognize and correct the problem: a different attenuation was clearly used on these nights for the b and v filters. Fortunately, the comparison was observed through both of these attenuations on the night JD 2446479 and I used these measurements as the needed calibration to correct the measurements of  CMa for -0 692 in b and -0 745 in v (note that HD 55876 has colours sufficiently similar to those of  CMa).

Finally, I should mention that there is also a problem with the second set of van Hoof's (1975) observations. He observed  CMa to be unusually faint: 4 2 in V. This is 0 15 fainter than the minimum observed by any other observer and in contradiction with one Fenstein's observation from the same night of V =4 01. One could speculate that  CMa = HD 53244 (V =4 12) was observed instead of  CMa by van Hoof at that time but the puzzle will probably remain unanswered since Professor A. van Hoof passed away a long time ago.

© European Southern Observatory (ESO) 1998

Online publication: May 15, 1998

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